The invention relates generally to steam turbines and more specifically to steam flow arrangements within the steam turbines to minimize thrust.
Today, large steam turbines are often used for large combined cycle power systems having a steam turbine and gas turbine, together driving an electrical generator as the load. Many arrangements for gas turbines and steam turbines in a combined cycle have been proposed. A combined cycle is an integrated thermal cycle, wherein the hot exhaust gas from a combustion gas turbine contributes heat energy to partially or wholly generate the steam used in the steam turbine.
A steam turbine is a mechanical device that extracts energy from pressurized steam, and converts the energy into useful work. Steam turbines receive a steam flow at an inlet pressure through multiple stationary nozzles that direct the steam flow against buckets rotationally attached to a rotor of the turbine. The steam flow impinging on the buckets creates a torque that causes the rotor of the turbine to rotate, thereby creating a useful source of power for turning an electrical generator or the like. The steam turbine includes, along the length of the rotor, multiple pairs of nozzles (or fixed blades) and buckets. Each pair of nozzle and bucket is called a stage. Each stage extracts a certain amount of energy from the steam flow causing the steam pressure to drop and the specific volume of the steam flow to expand. Consequently, the size of the nozzles and the buckets (stages) and their distance from the rotor grow progressively larger in the later stages. For cost and efficiency purposes, it is generally desired to extract the most energy possible before discharging the exhausted steam flow to a vacuum in a condenser.
In large power steam turbines, the number and diameter of the stages become massive. Usually, it is desired to separate the energy extraction process into two separate turbines, referred to as a high pressure steam turbine and a low pressure steam turbine. The high pressure steam turbine accepts the initial steam flow at a high pressure and exhausts into a low pressure steam turbine that continues the energy extraction process. The high pressure steam turbine must be constructed to withstand the greater forces created by the high pressure steam. The low pressure steam turbine must be larger to accommodate the large specific volume of the steam at reduced pressure.
Steam turbines may further be classified with regard to the action of the steam in conversion from heat to mechanical energy. The energy transfer may occur by an impulse mechanism, a reaction mechanism or a combination of the two. An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage.
In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference of the rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. Historically, full advantage has not been taken of the reaction mechanism in extracting energy from the steam turbine, in part because turbine performance was considered adequate and in in part due to difficulty in responding to increased axial thrust on the rotor shaft resulting from increased reaction forces on the moving blades.
Increased fuel costs and a desire by customers for improved steam turbine performance has raised interest in driving increased efficiency through a higher reaction output. For example, single flow HP-LP steam turbines are frequently used for desalination plants, where these plants are located in places where fuel is relatively cheap. Even so, with current fuel prices, performance is becoming an important parameter even for these applications. Performance expense for these type plants went from $300/kw to $800/kW in the last 2/3 years, highlighting the current emphasis on improved performance.
A conventional arrangement for a single flow high pressure-low pressure (HP-LP) steam turbine is illustrated in FIG. 1. A flow path for a HP-LP steam turbine may be defined as the steam flow among turbine units supported between a pair of journal bearings. In a single flow HP-LP steam turbine 5, the current orientation is to have the HP turbine 10 first followed by the LP turbine 20, both aligned in the same direction and connected by a vertical joint 25. The common rotor shaft 30 of the HP-LP turbine 5 may be supported by journal bearings 35 at opposing ends. Axial HP steam flow 50 passes through vertical joint 25 and axial LP steam flow 55 pass through the HP-LP steam turbine 5 in the same direction creating HP thrust 60 and LP thrust 65 resulting in an additive net thrust 70. Further, one large combined thrust bearing 40 may be provided may be provided at an end of the common rotor shaft 30 to absorb the combined net thrust 70 of the HP turbine 10 and the LP turbine 20. In many cases, the combined thrust bearing 40 is sized as large as is possible for the application.
The problem of large axial thrust was previously solved by using a large thrust bearing and low reaction levels in the steam turbine design. This is not a good performance combination as large thrust bearing means large bearing losses and low reaction means low steam path performance. Such configurations have none or very little performance room to improve.
If the steam path performance is to be improved, the major source of improvement left available is to increase stage reaction in either, or both, the HP and LP turbines. Increased stage reaction, however, leads to increased thrust loads necessitating greater thrust handling capability (reflected in greater size of the thrust bearing). In some applications with single flow HP-LP steam turbine units, current units already use the largest size special purpose bearing available. The size of the thrust bearings already restrict the performance of HP-LP single flow units forcing a low reaction steam path design around 5%.
Accordingly, there is a need to provide an arrangement for a HP steam turbine and a LP steam turbine combination to advantageously limit thrust, so an overall steam path efficiency may be improved by increasing stage reaction.